Scientists investigate past climates by studying substances that accumulate slowly over time. Trees, stalactites and stalagmites, peat, ice, lake sediments, ocean sediments, and corals all grow or accumulate in ways that reflect the environment surrounding them. Specifically, the isotopes within their structures fingerprint the conditions prevalent at the time of formation, be they wet, dry, cold, or warm.
But paleoclimate records aren’t limited to tree rings and the like. As scientists search for records that provide regional coverage, they’re starting to turn to some unusual sources: sinter in ancient Roman aqueducts, for example, or even piles of bat guano, excreted in caves around the world, accumulating for thousands of years.